| Abstract [eng] |
GaAsBi quantum well (QW) structures are one category of materials currently being studied as an active area for light sources operating in the near-infrared range. GaAsBi exhibits a low temperature sensitivity of the bandgap energy, a pronounced bandgap reduction with increasing bismuth incorporation and the potential to suppress non-radiative Auger recombination mechanisms. However, the growth of GaAsBi quantum structures with high emission efficiency is challenging. Bi incorporation into the GaAs lattice requires very low growth temperatures, which lead to a high defect concentration in the structures. In order to increase the photoluminescence (PL) intensity of GaAsBi QWs, various technological approaches are employed: the use of graded AlGaAs barriers improves carrier confinement and trapping efficiency, while annealing reduces the defect concentration. At the same time, thermal treatments can promote Bi segregation, which may lead to the formation of Bi quantum dots (QDs). To control the size of Bi QDs and to avoid structural degradation caused by enhanced Bi migration, ultrathin AlAs layers can be used as Bi-blocking barriers. The aim of this work was to investigate how annealing and barrier design affect optical and structural properties of GaAsBi/AlGaAs parabolic quantum wells (PQWs) with Bi QDs. In the first part of the thesis, in-situ and ex-situ annealed GaAsBi PQW structures were studied. Analysis of transmission electron microscopy micrographs showed that in-situ annealing improved structural quality, while PL measurements revealed reduced non-radiative recombination, enhancing the emission intensity and reducing thermal PL quenching. For the ex-situ annealing, samples were thermally treated in a rapid thermal annealing (RTA) furnace at 650 °C, 700 °C and 750 °C. It was found that the optimal RTA temperature was structure-dependent: previously in-situ annealed structure exhibited higher thermal stability and could benefit from higher RTA temperatures before the signs of degradation appeared. In the second part of this work, Bi redistribution and cluster formation induced by annealing treatments were investigated. It was shown that insufficient suppression of interlayer Bi diffusion leads to distortion of the QW structure and formation of Bi QDs. PL measurements revealed a broad emission band and a S-shaped temperature dependence of the PL peak, associated with carrier localization in Bi QDs of different sizes. In contrast, by introducing ultrathin AlAs layers and thereby effectively suppressing Bi diffusion, it becomes possible to control the size of formed QDs and tune their emission. In such structure, a narrow cathodoluminescence band at ~ 0.9 eV was observed and attributed to emission from Bi QDs with a size comparable to the QW width. In the third part of the thesis, the effect of additional ultrathin AlAs barrier layers on emission from GaAsBi QWs was investigated. It was found that introduction of AlAs barriers degrades the optical quality of the samples by defect formation. Structures containing AlAs barriers had stronger carrier localization, faster thermal PL quenching and overall weaker emission intensity. Fortunately, it was shown that these optical properties can be improved by reducing AlAs barrier thickness and employing additional in-situ annealing. Thus, this work demonstrates that annealing and barrier design strongly affect the optical properties of GaAsBi PQWs. Optimized annealing significantly enhances emission from GaAsBi QWs and promotes Bi QD formation, while Al-containing barriers enable control over the size of these QDs and their emission wavelength. |
